This invention relates to an advance mechanism for a mine roof support assembly.
A known type of mine roof support assembly is constituted by a plurality of roof support units positioned side-by-side along, for example, a longwall face. Each of the units has a roof bar supported on a floor sill by means of hydraulic props. The floor sill may be constituted by a pair of spaced-apart floor girders. In order to advance the assembly to follow the advance of the longwall face, the roof support units are advanced, individually or in groups, by means of hydraulic advance rams. During the advance of any given unit, its hydraulic props are relaxed so that its roof bar is not under load.
A known type of advance mechanism comprises a pair of hydraulic advance rams and a pair of resilient, generally parallel guide rods. The guide rods are interconnected at one end by means of a head-piece which is attachable to a conveyor extending along a longwall face. The other ends of the guide rods are attached to a floor girder of a roof support unit. The hydraulic advance rams are pivotally connected to the guide rods and act on said floor girders (See DE-OS No. 2 758 663)
This type of advance mechanism enables the roof support units to be aligned accurately during their advance movements, whilst the resilience of the guide rods permits the floor girders of the units to adapt to irregularities in the floor of the mine working. Such an advance mechanism can be positioned between an adjacent pair of roof support units, the guide rails of that advance mechanism being fastened to the adjacent floor girders of that pair of roof support units. In this case, the advance mechanism is called an external advance mechanism. Alternatively, each roof support has two floor girders, and a respective advance mechanism is provided for each roof support unit, the guide rails of each advance mechanism being fastened to the mutually-facing sides of the floor girders of the associated roof support unit. In this case, the advance mechanism is called an internal advance mechanism. In either case, the advance mechanisms of a roof support assembly, are used to advance the conveyor (or other longwall installation) in steps to follow up the advance of the working face as it is won. The advance mechanisms are then used to advance the roof support units in one follow-up step. This follow-up advance of the roof support units can be accomplished in two ways. Thus, the conveyor (or other longwall installation) can be used as an abutment for the follow-up step. Alternatively, the roof support units can be advanced individually, with the stationary roof support units providing an abutment for the follow-up movement. In this case, the advance of the roof support units is said to be independent of the conveyor (or other longwall installation).
This known type of advance mechanism has, however, a number of disadvantages. In particular, because of the arrangement of its hydraulic advance rams, when the roof support units are being advanced, only one ram of each advance mechanism is effective. This means that one of the roof support units associated with each advance mechanism must be advanced by the retraction of one of the hydraulic rams of that advance mechanism. This is disadvantageous because, during retraction, the annular working chamber of a ram is pressurized, and the area of this chamber is obviously smaller than that of the other (cylindrical) working chamber of that ram. Consequently, the advance force applied by this ram is relatively small, and this is disadvantageous in that this relatively small force must be used to advance the associated roof support unit which is very heavy. Moreover, the guide rods of the advance mechanisms are used for transmitting the forces used for advancing. Consequently, during advance movements, oppositely-directed forces are simultaneously applied to the guide rods. The resulting bending moments (and rotational moments) which are applied to the guide rods make it difficult to guide the roof support units accurately during their advance movements.
The aim of the invention is to provide an advance mechanism for a mine roof support assembly which does not suffer from the disadvantages of the known type of advance mechanism.